The present application relates generally to power systems and, more particularly, to power distribution systems and methods of operating a power distribution system.
Known electrical distribution systems include a plurality of switchgear lineups including circuit breakers that that are each coupled to one or more loads. The circuit breakers typically include a trip unit that controls the circuit breakers based upon sensed current flowing through the circuit breakers. More specifically, the trip unit causes current flowing through the circuit breaker to be interrupted if the current is outside of acceptable conditions defined by current and time thresholds.
For example, at least some known circuit breakers are programmed with one or more current thresholds (also known as “pickup” thresholds) that identify undesired current levels for the circuit breaker. If a fault draws current in excess of one or more current thresholds for a predetermined amount of time, for example, the trip unit typically activates the associated circuit breaker to stop current from flowing through the circuit breaker or initiates a timing sequence that eventually activates the circuit breaker if the threshold is exceeded for a predetermined amount of time. However, in power distribution systems that include a plurality of circuit breakers, a typical arrangement uses a hierarchy of circuit breakers. Large circuit breakers (i.e., circuit breakers with a high current rating) that are positioned closer to a power source than a plurality of lower current feeder circuit breakers feed the lower current feeder circuit breakers. Each feeder circuit breaker may feed a plurality of other circuit breakers, which connect to loads or other distribution equipment.
A fault may occur anywhere in the circuit breaker hierarchy. When a fault occurs, each circuit breaker that has the same fault current flowing through it may measure the same fault current differently as a result of varying sensor and circuit tolerances. When the fault occurs, the circuit breaker closest to the fault should trip to stop current from flowing through the circuit breaker. If a circuit breaker higher in the hierarchy, that is, closer to the source than the circuit breaker closest to the fault, trips, multiple circuits or loads will unnecessarily lose service.
To accommodate for the varying tolerances and to ensure that multiple circuit breakers do not unnecessarily trip based on the same fault current due to measurement variance, the current thresholds of at least some known circuit breakers are nested with each other to avoid overlapping fault current thresholds. For example, thresholds for circuit breakers at upper levels of the hierarchy typically are higher than the thresholds for circuit breakers at lower levels of the hierarchy to avoid overlapping thresholds. The nested fault current thresholds cause circuit breakers at higher tiers or levels of the hierarchy to have increasingly higher current thresholds. Accordingly, circuit breakers at higher tiers may not be able to detect fault currents that lower tier circuit breakers may detect. In this way, the circuit breaker closest to the fault will operate in response to the fault and will have a lower fault current threshold than upper level circuit breakers. If a fault occurs at a higher level in the hierarchy, for example, between a feeder and a branch or between a main breaker and a feeder, the system may have a reduced fault detection sensitivity because the circuit breakers at the higher levels of the hierarchy have higher fault current thresholds that may not detect a damaging fault current within the higher levels.